1. Field of the Invention
The present invention relates to a circuit board used in electrical and electronic equipment and a method for manufacturing the same. In particular, the present invention relates to a method for manufacturing a circuit board suitable for a power electronics field that uses a relatively large current.
2. Description of the Related Art
In recent years, the improvement in density and function of a semiconductor or the like has been emphasized with a demand for high performance and miniaturization of electronic equipment. This leads to a need for a small high-density circuit board, on which the semiconductor or the like is mounted. Thus, an important design consideration is thermal dissipation of the circuit board. To improve the thermal dissipation, a circuit board that can increase the thermal conductivity of an insulating substrate and suppress a local temperature rise better than a conventional printed circuit board made of glass-epoxy resin is desired. A metal-based circuit board is known to have higher thermal conductivity than that of the glass-epoxy resin. The metal-based circuit board includes a metal plate of copper or aluminum and a circuit pattern formed on one surface of the metal plate via an insulating layer. However, it is difficult to reduce the size and weight of the metal-based circuit board because the metal plate has a relatively large thickness. Moreover, the insulating layer should be made thinner to enhance the thermal conductivity of the circuit board, causing the problems of decreasing a withstand voltage and increasing a stray capacitance. A ceramic substrate and a glass-ceramic substrate have higher thermal conductivity than that of the glass-epoxy substrate. However, they use metal powder or a material obtained by firing the metal powder as a conductive material. Therefore, the wiring resistance becomes relatively high, which increases loss and Joule heat generated in using a large current.
A circuit board with high thermal conductivity that can solve the above problems and be manufactured in a similar process to that of a conventional glass-epoxy substrate is disclosed, e.g., in JP 10-173097A. FIGS. 7A to 7D show a method for manufacturing the thermal conductive circuit board. A slurry that includes an inorganic filler and a thermosetting resin is formed into a sheet-shaped thermal conductive mixture 71. Then, the thermal conductive mixture 71 is dried and sandwiched between metal foils 72, as shown in FIG. 7A. Next, the thermal conductive mixture 71 is cured by applying heat and pressure to form an insulating layer 73, as shown in FIG. 7B. Then, holes 74 are processed as shown in FIG. 7C, and connections are made between the layers by copper plating. Finally, as shown in FIG. 7D, a circuit pattern 75 is formed, resulting in a thermal conductive circuit board.
When a circuit board is produced in this manner, drilling is employed generally to form the holes. However, when the thermal conductive mixture includes a high proportion of inorganic filler to enhance thermal conductivity, a drill is worn significantly due to the hard inorganic filler in the insulating material. Moreover, chipping or the like occurs easily, and thus the hole quality often is degraded. If the drill is replaced at frequent intervals to prevent wear, the productivity is reduced greatly and the cost for processing holes is increased as compared with the general printed circuit board.
When holes are formed in the thermal conductive mixture with its thermosetting resin uncured, and then the thermal conductive resin mixture and the metal foils are bonded while applying heat and pressure for curing, the hole shape cannot be maintained because the thermal conductive mixture is softened by heating. This makes it difficult to form the holes, so that the subsequent plating is impossible.